Gas burning installations which are supplied with a gaseous fuel, e.g. natural gas-methane, propane, butane, generally comprise a gas supply line terminating in at least one nozzle from which the fuel gas is discharged, a combustion zone downstream of this nozzle, a duct connecting the combustion zone and/or the region of the nozzle with an air intake port at which this duct takes the combustion sustaining gas, generally air, at ambient pressure from the ambient atmosphere, and a flue, chimney or stack which the combustion products discharges.
It is desirable to maintain the air factor, i.e. the molar, weight or volume ratio of air to gas, substantially constant during the combustion process and at a predetermined optimum level designed to maximize the degree of combustion of the gas and the heat value produced thereby.
It has been proposed, in this connection, to provide a control element in the path of the air to the combustion site which is capable of varying the flow cross section and hence the rate at which the air is delivered to the combustion site. In a conventional system of this type, this flow control element is actuated by a force which is a parameter of the air flow, i.e. is a function thereof.
Atmospheric fuel gas installations are, for the purposes of this description, combustion units operating with the gas as a fuel and in which the combustion is carried out for heating purposes or for other purposes utilizing generally an open combustion chamber to which the air is supplied via a duct of the aforementioned type without blowers or other forced air mechanisms upstream of the intake. The air can be drawn from the intake through the duct by the natural draft of the system.
In a system in which the air control element responds to a force which is a function of the air flow parameter, generally, and as described in German patent document--Utility Model DE-GM No. 79 08 061, the only forces acting upon this element are those of unavoidable friction, gravity and the force which is a function of the air flow parameter mentioned previously.
Such devices have been found to be effective with most gas consumers of the aforedescribed type, since such units generally have constant loads.
For multistage burner installations, burner installations the outputs of which are varied frequently or to a high degree, and other systems in which generally constant conditions cannot be maintained, the control by such an element of the air flow to the combustion site does not maintain the air/fuel gas ratio constant or sufficiently constant. For example, in many cases the air/fuel gas ratio decreases as the fuel gas throughput is increased because conventional devices of the type described tend to maintain the air flow rate constant and independent from the fuel gas throughput. This variation of the ratio, of course, adversely affects the efficiency of the apparatus and inevitably means that the efficiency at partial or low loading will be significantly less than the efficiency at full load or under nominal operating conditions of the apparatus.
It has been proposed (see the Institution of Gas Engineers, Communication 108, 1979, page 17, FIG. 10) to provide air-displacement type burner installations, using a blower which drives the combustion air through a duct, to control the air factor (ratio of combustion air to fuel gas) so as to maintain this ratio practically constant under all operating conditions by providing a membrane control unit. This unit is responsive to the pressure of the air ahead of the combustion site and controls a valve for the fuel gas so as to maintain the ratio constant.
However, this arrangement is not appropriate of being applied to atmospheric air combustion installations with varying load requirements and variable flue drafts.